bypass upstream proxy’s forbidden website by set up local proxy, why it works?

Question

Proxy server that restricted specific IPs to specific domains. I can bypass theses blocked websites by setup my own local proxy server while using the above proxy server as upstream. What happened? Why it worked?

Context:

• In my company, we use internal network for work, there're some security policies like block external storage cloud, message, social media platform, etc.
• We have static IP, DNS. To connect to outside internet, we must through a proxy server let's call proxy X. Proxy X controlled our connections.
• I want to access these restricted websites.

What I've done

I installed Squid as my local proxy, and using proxy X as upstream proxy. Let's call my local proxy as proxy Y, so that all request that go through proxy Y will go through proxy X. This is the only configuration that I made.

What happens

• when using proxy Y, I can access to these blocked websites.
• from what I saw, in my machine, if I provided hostname (to using local proxy like http://<host-name>:3128) like loopback address localhost(::1), 0.0.0.0, 127.0.0.1 or my-computer(10.0.0.x)(another adapter), basically everything but not hostname resolved to my static IP, eg:10.60.100.x will be able to access to these blocked websites.
• the static IP as said above can be generalized to IP that in 10.60.100.0/x subnet

My guess

• I thought maybe they've config ACL for this subnet and deny access to these blocked domains.
• By using maybe different header when send request to these blocked websites through proxy X by proxy Y, I can access them without restricted. If it's true, what could header be look like? Can I use different method without setup proxy Y.